At the gleno-humeral joint in the shoulder, the head of the humerus (which is the bone of the upper arm), articulates against the glenoid fossa for upper arm movement. The articulating surfaces are normally separated by cartilage which reduces friction between the articulating surfaces. The head (designated GG in FIG. 5) is held into place and its stability maintained by ligaments and muscle tissue (collectively designated HH in FIG. 5) surrounding the joint.
Damage to, or deterioration of these bones or the cartilage separating them often necessitates implantation of prosthetic glenoid and humeral components. Many candidates for shoulder prostheses suffer from rotator cuff arthropathy, a condition caused by disease, injury, or aging in which the muscle tissue of the shoulder is damaged or degenerated. Because stability of the shoulder joint depends solely on the integrity of the surrounding muscle tissue, these patients lack the tissue which is essential for holding the shoulder joint together.
The designs of some existing shoulder prostheses are directed towards solving the problem of unstable shoulder joints. Many involve mechanically linking the humeral component to the glenoid component, a procedure which severely limits the range of motion of the shoulder.
Ball and socket devices similar to hip prostheses are sometimes used in the shoulder. In typical ball and socket devices, a humeral component is provided which has a stem fixed inside the intermedullary canal of the humerus, and a spherical ball connected to its distal end. The spherical ball serves as a substitute for the patient's humeral head, which is removed prior to implantation of the humeral component. A cup attached to the glenoid fossa provides a surface upon which the ball articulates during arm movement.
The ball and socket type of shoulder implants are used for patients who, because of arthritis, have suffered from serious deterioration of the cartilage. Because the cartilage in a healthy shoulder provides a smooth surface against which the humeral head articulates, the humeral head of a patient having deteriorated cartilage articulates directly against the glenoid and thus suffers from serious pain. The ball and socket implants eliminate the pain by providing smooth articulating surfaces in the shoulder. However, these implants are not suitable for patients having rotator cuff arthropathy, because they provide no structure to keep the humeral head from migrating away from the articulating surface of the socket. Candidates for conventional ball and socket joints are patients who do not have damaged rotator cuffs and who therefore retain the tissue needed to prevent migration of the humeral head.